The invention relates to thin layers deposited on a transparent glass substrate to modify their optical characteristics. In particular, the invention concerns thin layers interposed between the glass substrate and another thin functional layer, in particular a solar protection or low-emission layer.
It is well known that the presence of a low-emission or solar protection layer, in particular those with the most suitable thicknesses and refractive indexes, both with respect to efficiency and to industrial application, causes parasitic colorations, in particular in reflected light. These colorations which occur in the form of areas of iridescence must be fully suppressed for conventional uses, and most particularly for clear glasses where their presence is most detrimental to the appearance of those glazing units comprising such glasses.
In order to prevent these phenomena, it is well known to interpose a “sub-layer”, the refractive index and thickness of which are selected to attenuate or practically eliminate these areas of iridescence, between the functional layer and the glass substrate. Theory allows the appropriate values for these parameters to be determined precisely. However, in practice some difficulties are encountered in the implementation because of the many technical requirements which industrial production must meet to produce these sub-layers in an efficient and economical manner.
While various solutions have been proposed for the production of these sub-layers, the most widely used is that of silicon-based sub-layers. The most advantageous methods of deposition from the point of view of expense for the formation of these sub-layers are based on pyrolysis, and most particularly gas-phase pyrolysis (CVD). In these methods the deposition of silicon working from gaseous precursors such as silanes is generally conducted directly on the glass ribbon continuously while this is being produced. The temperature conditions of the glass enable the reaction of pyrolysis of the precursors which are brought into contact with this. However, the choice of proceeding in these conditions implies that there are set requirements associated with the special features of the assembly into which the deposition operation has just been integrated.
The deposition of sub-layers in the conditions which have just been outlined must be conducted quickly. The ribbon of glass to be coated passes under ducts which bring the precursors into contact with it. The contact time of the gas and glass, irrespective of the arrangements of the plant, is necessarily limited. Therefore, highly reactive precursors must be used to achieve the thickness of deposition during this very short period. The precursors of silicon layers have a very high reactivity in temperature conditions such as those of the ribbon of glass exiting from the molten tin bath.
However, previous methods of forming silicon-based sub-layers have some disadvantages. In particular, the precursors of silicon cause rapid fouling of the plant.
The deposition ducts are disposed in direct proximity to the ribbon of glass to ensure good contact and thus a favourable output from the reaction. The distance separating the ribbon of glass from the duct or ducts is generally only a few mm. In these conditions, parasitic deposits of reagents on the structures in the vicinity of the duct rapidly reach dimensions which necessitate cleaning, otherwise the presence of parasitic deposits causes disruptions in the flows of gas, which lead to irregularities in the layer. If fouling continues for long the glass may even rub against the concretions formed which leads to unacceptable marks and scratches. In the present state, cleaning constitutes an interruption in production at regular intervals, and this is even more detrimental to the good function of the operation since these cleaning operations are very frequent.
Moreover, the sub-layer must have well controlled properties, in particular the refractive index, in order to take into account the special features of the layers from which they eliminate the disadvantages described above. The precise characteristics of the silicon-based layers are difficult to control. Various proposals have been made to achieve a well defined refractive index without having come to a perfectly satisfactory method.